TECHNIQUES FOR LINE NARROWING
Techniques for decreasing the bandwidth of the output of lasers are well known. Several such techniques used on excimer lasers are discussed by John F. Reintjes at pages 44-50 in Laser Handbook, Vol. 5, North-Holland Physics Publishing, Elsevier Science Publishers B.V. These techniques include the utilization of gratings, including echelle gratings for wavelength selection. All frequency selective elements, including gratings, depend on angular dispersion for their selectivity. Therefore, the spectral revolution of these frequency selective systems is improved by reducing the beam divergence. Divergence can be reduced with the use of small apertures. One well known technique is to reduce the divergence of the beam by magnifying it with a beam expander, such as a telescope upstream of the frequency selection element. Beam expansion reduces the divergence angle by the magnification factor of the beam expander. Prisms inserted between the gain medium and the frequency selecting element have been used to magnify the beam in a single direction. Unstable resonators are well known, and it is known that unstable resonators may be configured to provide reduced beam divergence.